In most large commercial aircraft, the maximum rotation angle of the aircraft during takeoff and landing is limited by a minimum permissible clearance between a rear under portion of the fuselage and the ground. It is known that the takeoff and landing performance of a given aircraft can be enhanced by providing a longer main landing gear about which the aircraft rotates to achieve a nose-up attitude, thereby increasing the maximum rotation angle of the aircraft. However, one of the objectives of aircraft design is to configure the landing gear so that the aircraft fuselage is essentially horizontal during ground operations and has an appropriate sill height for ground servicing. The maximum sill height that is acceptable is dictated by the height of ground equipment that must interface with the aircraft, and thus is generally fixed. In many cases, the maximum allowable sill height is less than what would be desirable from an aircraft performance standpoint, and therefore, merely lengthening the landing gear is not a viable approach to achieving increased maximum rotation angle. Further, landing gear length must be minimized to keep weight to a minimum and to facilitate the stowing of the gear during flight, and hence a wholesale lengthening of the landing gear is undesirable.
In view of the above considerations, efforts have been made to develop variable-length landing gear capable of assuming a length that is suitable for stowing within the aircraft, and for ground operations while the aircraft is on the ground and stationary, and further capable of assuming a greater length during takeoff and landing operations. One such type of variable-length landing gear, to which the present invention relates, is the semi-levered landing gear (SLG). In a typical SLG, a wheel truck is formed by a bogie beam supporting forward and aft wheels at forward and aft ends thereof, and a main strut of conventional design is pivotally connected to the bogie beam at a main pivot between the forward and aft wheels. An additional mechanical linkage is connected at an upper end to the main strut and at a lower end to the bogie beam at an auxiliary pivot spaced from the main pivot for controlling positioning of the bogie beam. The additional mechanical linkage enables the bogie beam, under certain conditions, to pivot about the auxiliary pivot rather than the main pivot. In this manner, when the aircraft approaches the end of a takeoff roll and begins to rotate for liftoff, the bogie beam can be placed in a tilted orientation with the forward wheels off the ground with the aid of the additional mechanical linkage, which prevents the bogie beam from rotating to a horizontal orientation. With the wheel truck in this tilted position, the effective length of the landing gear is increased relative to its length when all wheels are on the ground. The aircraft can then rotate to a higher pitch attitude, with the same tail clearance, thus achieving improved takeoff performance.
Existing semi-levered landing gears can be unsatisfactory for various reasons. In some types of SLG configurations, such as that disclosed in U.S. Pat. No. 4,892,270 to Derrien et al., the additional mechanical linkage comprises a passive torque link assembly whose only function is to lock up when the main strut and the bogie beam assume particular positions, namely, when the bogie beam is tilted and the main strut is relatively uncompressed as it is on initial touchdown and at liftoff. These types of SLG devices require an additional actuator or spring device for placing the bogie beam in the tilted position for landing. Where the means for tilting the bogie beam is a passive spring device as in the Derrien '270 patent, stowing of the landing gear in the aircraft can be complicated by the lack of ability to reposition the bogie beam in a more-appropriate position for stowage.
One method that has been used to reposition the bogie for stowage with this type of SLG employs a shrink-link main strut that is operable to shorten as the landing gear is retracted into the wheel well, thereby changing the geometry of the SLG link and bogie so that the gear can be stowed. A disadvantage of this approach is that the shrink-link main strut is of considerably greater complexity and weight than a conventional main strut, thereby adding cost and weight to the aircraft.
Accordingly, some SLG configurations employ an active device connected between the main strut and the bogie beam for placing the bogie beam in a tilted position. For example, published UK Patent Application No. GB 2,101,542A by Putnam et al. discloses an aircraft undercarriage unit having a variable length oleopneumatic strut connected between the main strut and an aft end of the bogie beam. The variable length strut is hydraulically actuated to extend so as to tilt the bogie beam during takeoff. After takeoff, the variable length strut is contracted to position the bogie beam substantially horizontal to facilitate stowage of the gear. A major problem with Putnam's landing gear design is that it is incapable of maintaining equal loading on all main gear wheels during braking at all aircraft weight and aerodynamic lift conditions, because the variable-length strut is always active to exert a force on the bogie tending to tilt the bogie, which occurs when the overall load on the landing gear drops to a sufficiently low level. The result is that Putnam's landing gear would require larger brakes, and larger wheel wells to contain them, in order to assure adequate braking capacity during landing rollout or refused takeoff, thus incurring a significant penalty to the aircraft design in terms of weight and wheel well volume.
Another type of main landing gear is disclosed in UK Patent 1,510,554 by Faithfull. The Faithfull patent states as its object and advantage the capability of effectively lengthening the landing gear at touchdown to provide improved shock absorbing characteristics during landing at relatively high descent rates. The landing gear purportedly achieves this object by the use of an additional oil-filled cylinder, functioning only as a passive damper, pivotally attached to the front of the bogie beam and the upper stationary part of the main shock strut. In preparation for landing, the bogie is placed into a tilted position via a positioning device that is separate from the oil-filled cylinder. In this tilted position, the oil-filled cylinder is in a compressed condition. Upon touchdown and landing rollout, the bogie begins to rotate toward a horizontal position, thus causing the oil-filled cylinder to be extended until it reaches its maximum length. The maximum length of the oil-filled cylinder is such that the bogie cannot rotate to a fully horizontal position during the initial portion of the landing rollout, and hence the effective length of the landing gear is greater during this initial portion of the rollout.
Faithfull does not claim that his device is capable of providing improved takeoff performance through effective gear lengthening. Moreover, Faithfull's device would prevent the most advantageous positioning of the bogie for stowage of the gear in the aircraft. In order to stow the landing gear in most aircraft, the bogie advantageously should be placed in an approximately horizontal position (on some large commercial aircraft, the bogie must rotate past horizontal into a pitch-down attitude of as much as 15 degrees) with the main strut fully extended, this orientation enabling the wheel well size to be kept to a minimum. However, Faithfull's oil-filled cylinder has a maximum extension selected such that the bogie is tilted into a pitch-up attitude when the main strut is slightly compressed on landing. Thus, the oil-filled cylinder simply cannot extend sufficiently to position the bogie horizontal with the main strut fully extended. If the oil-filled cylinder disclosed in Faithfull were modified to provide sufficient stroke to accommodate the bogie stow position, it would be incapable of providing the semi-levered function on landing. Furthermore, if the stroke length were selected to provide effective semi-levered function on takeoff, then the bogie would assume a pitch-up attitude for stowage, which would require a very large wheel well. Thus, Faithfull's device is incapable of simultaneously providing semi-levered function and enabling an optimum positioning of the bogie for stowage.
A main landing gear configuration disclosed in U.S. Pat. No. 4,749,152 is said to provide an effectively longer landing gear at takeoff, but requires a very complex main strut having multiple main strut cylinders, some with offset loading. This main strut would result in a very heavy landing gear relative to a conventional main strut. Additionally, the landing gear in the '152 patent requires a shrink-link mechanism to reposition the bogie for stowage. Furthermore, the multiple-cylinder design results in sliding surfaces that cannot be inspected without major disassembly, thus increasing maintenance costs. Finally, another disadvantage of the gear design disclosed in the '152 patent is that all of the purported functions of the gear, including semi-levered action at takeoff, absorption of energy at touchdown, equal wheel loading during ground roll, and bogie repositioning, are provided by the main strut. This may hamper the optimization of each of these functions because of space and geometry limitations of the design.